diff --git a/magit.el b/magit.el
index ff33f7c0..f47356e5 100644
--- a/magit.el
+++ b/magit.el
@@ -401,13 +401,11 @@ Many Magit faces inherit from this one by default."
 
 (defun magit-get-top-dir (cwd)
   (let ((cwd (expand-file-name cwd)))
-    (and (file-directory-p cwd)
-	 (let* ((default-directory cwd)
-		(magit-dir
-		 (magit-git-string "rev-parse" "--git-dir")))
-	   (and magit-dir
-		(file-name-as-directory
-		 (or (file-name-directory magit-dir) cwd)))))))
+    (when (file-directory-p cwd)
+      (let* ((default-directory cwd)
+             (cdup (magit-git-string "rev-parse" "--show-cdup")))
+        (when cdup
+          (file-name-as-directory (expand-file-name cdup cwd)))))))
 
 (defun magit-get-ref (ref)
   (magit-git-string "symbolic-ref" "-q" ref))